1. Field of the Invention
This invention relates to water purification systems and, more particularly, to water purification systems using filtration and ultraviolet radiation apparatus to purify the water.
2. Description of the Prior Art
It is known to use ultraviolet (UV) radiation to disinfect drinking water. The United States Department of Health, Education and Welfare Public Health Service recognizes that ultraviolet radiation is a means for meeting the bacteriological requirements of the Public Health Service Drinking Water Standards. Studies have indicated that ultraviolet radiation at a level of 2,537 Angstrom units applied at a minimum dosage of 16,000 micro-watt-seconds per square centimeter at all points throughout a water disinfection chamber is adequate to purify water for drinking. Industry has set its own standard of 30,000 micro-watt-seconds to account for any losses that may occur in the chamber. If one uses a UV lamp that has a guarantee of maintaining 80% power over 9,000 hours (one year) then setting a standard of 38,000 micro-watt-seconds per square centimeter would insure that the lamp would require replacement no more than yearly, while adequately sterilizing water flow exposed to the UV lamp.
Many rural areas in the United States and throughout the world do not have high water pressure and do not have a reliable water source. For instance, people who depend on cisterns or well water may have good water today and contaminated water tomorrow. Many of these systems use gravity as the source of water pressure or have well pumps that are only a slight improvement.
In U.S. Pat. No. 4,968,437 to Electrolux a sterilization system employs two plastic tubes at least six feet in length having a cross-sectional area of about 0.2 square inches. The tubes are wrapped spirally, interleaved around an ultraviolet-light-producing lamp, and connected in series with a filter between them. The apparatus disclosed in the Electrolux '437 patent may be suitable for some applications, but there are applications where it is not practical.
First, if the water being treated is turbid the first plastic tube will be ineffective because there is no pre-filter. The UV light will not be able to pass through the water and will not be entirely effective in destroying all microorganisms. Moreover, because there is no pre-filter, material passing through the first plastic tube can accumulate on the inside surface of the tube. The accumulation can render the first tube ineffective over time by restricting the transmittance of the UV radiation through the tube. This would require frequent inspection and cleaning/replacement of the tube system.
In addition, the transmission characteristics of plastic for ultraviolet radiation are significantly lower than those of quartz or high-silica glass recommended by the U.S. Public Health Service. Thus, the ultraviolet lamp would need to be quite large to overcome the losses built into the system. The absorption of the UV radiation by the plastic is called solarization and becomes worse as the plastic ages, producing darkening of the material and increasing the absorption rate of UV radiation.
Another problem with an Electrolux-type system is that the water must travel through at least 12 feet of tubing which is approximately 0.2 inches in cross-sectional area, causing significant losses in water pressure. The filter adds to the pressure losses so that the flow rate would likely drop to a point that makes the unit undesirable for low pressure applications. Moreover, the size of the Electrolux '437 unit and hookup method make it impractical for a counter-top unit.
Another prior art system shown in FIG. 1 utilizes a radial-flow carbon cartridge and a ultraviolet lamp in the center. In this system, the water can flow at any point along the carbon cartridge into proximity with the ultraviolet lamp and then to the output. Thus, the flow of at least part of the water can substantially bypass the ultraviolet lamp at or near the input to the filter, thereby contaminating the water flowing out of the filter.
As shown in FIG. 1, the water enters at input port 1 and fills up the housing. It then travels through the filter 2 and passes the ultraviolet lamp 3 before exiting at output port 4. Because the water will take the path of least resistance, some of it will cross the filter 2 at the top and have a shorter path past the ultraviolet lamp 3 to the exit port 4.
Another problem with this prior art filter is that the ultraviolet light is radiated directly on the plastic housing of the filter medium. By continued exposure to UV light, the plastic housing will break down and release some of its component material into the water.